Conventionally, a Schottky diode includes a heavily-doped semiconductor substrate, typically made of single-crystal silicon. A second layer covers the substrate, The second layer, called the drift region, is less heavily-doped with a material having carriers of the same conducting type as the substrate. A metal layer or a metal silicide layer forms a Schottky contact with the lightly-doped drift region and forms the diode anode.
Two opposing constraints arise when forming a unipolar component such as a Schottky diode. In particular, the components should exhibit the lowest possible on-state resistance (Ron) while having a high breakdown voltage. Minimizing the on-state resistance imposes minimizing the thickness of the less doped layer and maximizing the doping of this layer. Conversely, to obtain a high reverse breakdown voltage, the doping of the less doped layer must be minimized and its thickness must be maximized, while avoiding the creation of areas in which the equipotential surfaces are strongly bent.
Various solutions have been provided to reconcile these opposite constraints, which has led to the development of trench MOS-capacitance Schottky diode structures, which are referred to as Trench MOS Barrier Schottky (TMBS) diodes. In an example of such structures, conductive areas, for example, heavily-doped N-type polysilicon areas, are formed in an upper portion of a thick drift region less heavily N-type doped than an underlying substrate. An insulating layer insulates the conductive areas from the thick layer. An anode layer covers the entire structure, contacting the upper surface of the insulated conductive areas and forming a Schottky contact with the lightly-doped drift region.
When reverse biased, the insulated conductive areas cause a lateral depletion of into the drift region, which modifies the distribution of the equipotential surfaces in this layer. This enables increasing the drift region doping, and thus reducing the on-state resistance with no adverse effect on the reverse breakdown voltage.
FIG. 1 is a simplified, partial view of a conventional TMBS Schottky diode or rectifier. The diode is formed from a heavily-doped N-type silicon wafer 1 on which is formed a lightly-doped N-type epitaxial layer 2. Openings are formed in this epitaxial layer, which may be, for example, trench-shaped. Conductive regions 3 are formed in the openings, which are made, for example, of doped polysilicon. An insulating layer 4 is interposed between each conductive region and the walls of the corresponding opening (e.g., trench). The insulating layer 4 may be formed, for example, by thermal oxidation and the opening may be filled with polysilicon by conformal deposition, followed by a planarization step. After this, a metal, for example, nickel, capable of forming a silicide 5 above the single-crystal silicon regions 6 above the polysilicon filling areas, is deposited. Once the silicide has been formed, the metal which has not reacted with the silicon is removed by selective etch. After this, an anode metal deposition 7 is formed on the upper surface side and a cathode metal deposition 8 is formed on the lower surface side.
The use of a trench structure instead of a planar structure creates about an additional 20V of blocking capability (for a trench that is about 2 microns in width). This increase results from the relocation of the peak electric field from the epitaxial layer surface to the bottom of trenches and redistribution of electric field in silicon. The significance of this additional blocking capability is reduced for higher blocking voltages. For example, 20V represents a 25% increase in blocking voltage for an 80V device but only represents an 11% increase for a 180V device.